This invention relates to a vapor detector for detecting combustible vapors such as gasoline in marine propulsion apparatus.
In marine propulsion apparatus, the internal combustion engine is housed within a suitable enclosure. The enclosure not only protects the operating personnel from the operating components and the engine from the external environments, but provides a high degree of sound proofing. The enclosures are generally therefore made relatively complete to establish maximum sound deadening.
When the engine is not operating, and particularly before initial engine start-up, gasoline vapors and the like may accumulate within the enclosure resulting in an explosive condition. Generally, in inboard engines mounted within a bilge chamber or within an enclosure of an inboard-outboard drive, fan units are provided for exhausting of the enclosure immediately before the starting of the engine. In addition, vapor sensitive devices have been mounted in the enclosure for monitoring of the presence of any combustible gases and generating of a suitable visible and/or audible alarms in the presence of dangerous levels of combustible vapors. A satisfactory vapor sensing element is shown, for example, in U.S. Pat. No. 3,045,198. The element is a semi-conductor which has resistance with increases within the presence of combustible vapors. The element has generally been connected in a voltage dividing network and generates a voltage signal which is connected to a multistage transistor amplifier connected to drive various alarm means in parallel. Although providing a satisfactory system, the conduction and gain of transistors varies with temperature in such a manner that an erroneous alarm condition may be signaled. As a practical matter, the amplifier is also driven from the conventional supply battery, the output of which may vary over a reasonable range. As the system employs a voltage dividing network, the alarm set point also varies with the supply voltage. Further the combination of the sensor with the amplifier does not provide a fail safe operation for both shorted and open circuit conditions. Further, where a heavy repeater load is also driven from the output of the amplifier, the output transistor may malfunction and fail to produce an alarm. The inventor's analysis has indicated that the amplifying circuit with a finite gain tends to slowly turn on as the resistance of the sensor element increases over the period of time associated with the increase in the combustible vapors. The repeater load connected to the output may hold the transistor only to fifty percent of saturation, resulting in a maximum heat dissipation within the output transistor. If a heavy load, such as a repeater, is connected in the output circuit, the dissipation rating of the output transistor may be exceeded and the transistor destroyed.